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Chapter 5
Actions during service
DOI: 10.1201/b12914-5
5.1 MechAnicAl Actions
5.1.1 Direct loading and impact
Concrete structures or structural elements are designed to take up external
mechanical loading. In modern design codes based on ultimate limit state
design, it is considered that concrete in tension will crack, leaving only
the reinforcement to carrying the tensile loading. This does not mean that
concrete structural elements under service load will always show cracks.
However, it does mean that if cracks occur in concrete structures, the
structural load-bearing capacity of the structure is not immediately at risk.
Of course, excessive cracking in concrete structures is neither generally
expected nor accepted. And even small cracks could impair long-term concrete durability because of the accelerated ingress of aggressive substances.
Cracks due to direct mechanical loading can be easily recognized by looking at the crack pattern. Different load conditions will lead to typical crack
patterns, as illustrated in Figure 5.1. In laboratory conditions, when the structural performance of concrete elements is experimentally tested, a fully developed crack pattern can be obtained when loading up to failure. However,
in real practice, when the concrete element is well designed, the number of
cracks and the crack widths will be rather limited. Excessive cracking might
indicate inappropriate design or unexpected overloading of the structure.
Crack widths will depend on several parameters such as the bond between
concrete and steel reinforcement, cover thickness, distance between reinforcing bars, and diameter of the reinforcing bars. Although considerable agreement exists concerning the principles of crack simulations and estimations,
different codes will typically predict different crack widths. Differences also
exist in prescribed maximum allowable crack widths according to different
design codes. For reinforced concrete, crack widths up to 0.3 mm are widely
accepted in cases of non-severe exposure conditions. In the case of a more
aggressive environment, allowable crack widths are typically reduced to
0.2 or even 0.1 mm.
Chapter 5
Actions during service
DOI: 10.1201/b12914-5
5.1 MechAnicAl Actions
5.1.1 Direct loading and impact
Concrete structures or structural elements are designed to take up external
mechanical loading. In modern design codes based on ultimate limit state
design, it is considered that concrete in tension will crack, leaving only
the reinforcement to carrying the tensile loading. This does not mean that
concrete structural elements under service load will always show cracks.
However, it does mean that if cracks occur in concrete structures, the
structural load-bearing capacity of the structure is not immediately at risk.
Of course, excessive cracking in concrete structures is neither generally
expected nor accepted. And even small cracks could impair long-term concrete durability because of the accelerated ingress of aggressive substances.
Cracks due to direct mechanical loading can be easily recognized by looking at the crack pattern. Different load conditions will lead to typical crack
patterns, as illustrated in Figure 5.1. In laboratory conditions, when the structural performance of concrete elements is experimentally tested, a fully developed crack pattern can be obtained when loading up to failure. However,
in real practice, when the concrete element is well designed, the number of
cracks and the crack widths will be rather limited. Excessive cracking might
indicate inappropriate design or unexpected overloading of the structure.
Crack widths will depend on several parameters such as the bond between
concrete and steel reinforcement, cover thickness, distance between reinforcing bars, and diameter of the reinforcing bars. Although considerable agreement exists concerning the principles of crack simulations and estimations,
different codes will typically predict different crack widths. Differences also
exist in prescribed maximum allowable crack widths according to different
design codes. For reinforced concrete, crack widths up to 0.3 mm are widely
accepted in cases of non-severe exposure conditions. In the case of a more
aggressive environment, allowable crack widths are typically reduced to
0.2 or even 0.1 mm.